In a medium bowl, stir together flour, sugar and cocoa. Using a pastry blender, cut in butter until crumbly.

In a small bowl, combine egg yolk and 2 tablespoons of water.

Gradually add egg mixture to flour mixture until combined. If necessary, add additional water until dough starts to cling together. Knead dough until smooth. Form dough into a ball. Cover and chill for approximately 1 hour or until dough is easy to handle.

Preheat oven to 350 degrees.

Divide dough into 12 balls. Press dough evenly into the bottom and up the sides of 12 ungreased muffin cups. Bake for 15 minutes. Cool completely on wire rack.